Bifocal and trifocal contact lenses are commonly used to treat presbyopia, a condition in which the eye exhibits a progressively diminished ability to focus on near objects. Human beings become presbyopic due to aging, and the effect typically becomes noticeable starting at about the age of 40-45 years, when they discover they need reading glasses. Presbyopic individuals who wear corrective lenses may then find that they need two separate prescriptions, preferably within the same bifocal lens, one for reading (near) and another for driving (distance). A trifocal lens further improves vision at intermediate distances, for example, when working at a computer. An intraocular lens (IOL) is an artificial replacement lens that may be used as an alternative to a contact lens or eyeglasses. An IOL is often implanted in place of the natural eye lens during cataract surgery. An intracorneal lens (ICL) is an artificial lens that is implanted into the cornea.
Conventional corrective optics are typically refractive lenses, meaning that they bend and focus light rays reflected from an object to form a focused image of the object on the retina. The bending of the light rays is dictated by Snell's law which describes the degree of bending that occurs as light rays cross the boundary of two materials with distinct indices of refraction.
Diffractive lenses have a repeating structure that may be formed in the surface of an optical element by a fabrication method such as, for example, cutting the surface using a lathe that may be equipped with a cutting head made of a hard mineral such as diamond or sapphire; direct write patterning using a high energy beam such as a laser beam or electron beam or a similar method of ablating the surface; etching the surface using a photolithographic patterning process; or molding the surface. The diffractive structure is typically a series of concentric annular zones, which requires each zone to become progressively narrower from the center to the edge of the lens. There may be, for example, 20-30 zones between the center and the edge of the lens. The surface profile within each zone is typically a smoothly varying function such as an arc, a parabola, or a line. At the outer periphery of each zone there is a discrete step in the vertical surface profile, the step height typically measuring about 0.5-3 microns. The resulting surface structure acts as a circularly symmetric diffraction grating that disperses light into multiple diffraction orders, each diffraction order having a consecutive number, zero, one, two, and so forth.
“Diffraction efficiency” refers to the percentage of incident light power transmitted into each of the various diffractive orders comprising the diffraction pattern at the focal plane. If the zones have equal surface areas and are radially symmetric, they focus light of different diffraction orders onto the optical axis of the lens, each diffraction order having its own distinct foci. Thus, the diffractive lens acts as a multifocal lens having many discrete foci. For example, a diffractive bifocal lens simultaneously provides sharp retinal images of objects at two different distances, as well as two corresponding out-of-focus images. The human visual system has the ability to select from among the different retinal images, thereby enabling simultaneous multifocal vision using a single diffractive lens.
Diffractive lenses may be used as contact lenses and IOLs for correcting presbyopia. In such an application, the lens comprises one refractive surface and one diffractive surface. In practice, the light energy passing through a diffractive lens is typically concentrated into one, two, or three diffractive orders, while contributing an insignificant amount of light energy to other diffractive orders. With respect to diffractive corrective lenses, for example, a high diffraction efficiency for the zeroth order connotes a greater improvement in visibility at far distances. The amount of optical energy directed into each diffraction order is dictated by the zonal step heights. A lens designer may choose, for the diffractive surface features of a bifocal lens, step heights so as to introduce, for example, a one-half wavelength phase change between adjacent zones, which directs approximately 40% of the incident light into the zeroth diffraction order corresponding to distance vision, and 40% into the positive first diffractive order, corresponding to near vision. The remaining 20% of the incident light in a conventional bifocal lens is directed to other diffraction orders that are not useful for vision.
Existing designs for multifocal intraocular and contact lenses use either refractive optics, a combination refractive/diffractive design, or diffractive lenses that direct light into a single diffractive order. For example, U.S. Pat. No. 5,344,447 to Swanson, discloses a trifocal IOL design that enhances distance vision using a combination lens having a refractive surface and a diffractive surface. Each diffractive zone in this case corresponds to a binary step. This lens distributes light approximately equally between the positive first, zeroth, and negative first diffraction order. However, a drawback to this configuration is that excess light is directed into other higher diffractive orders, reducing visual quality. Furthermore, this configuration makes the power of the underlying carrier lens more difficult to predict because distance vision is dictated by a combination of the lens' refractive power with the diffractive power of the minus one diffractive order. None of the existing alternatives succeeds in directing enough light into a diffractive order that corresponds to an intermediate focal distance, and therefore trifocal contact lenses and IOLs fail to perform equally well throughout the full focal range. For example, U.S. Pat. No. 7,441,894 to Zhang et al. discloses a trifocal intraocular lens having diffractive zones of varying areas capable of directing about 25-28% of incident light into the near and far foci, but only about 10% of the incident light is directed into the intermediate focus.